In the exploitation of hydrocarbon reservoirs, use is made of well logging to determine the amount of recoverable hydrocarbons. Logging tools such as density, neutrons and resistivity logs, have been developed for measurement of reservoir properties such as porosity and water and hydrocarbon saturation of pore space. These tools are widely utilized in the oil industry. However, in order to accurately determine porosity using these tools, the lithology of the rock must be known. Many other rock and fluid properties, such as salinity, cementation factor, saturation exponent, and shaliness, must be known to determine hydrocarbon saturation using a resistivity log. In addition, methods are not known for estimating pore size or permeability in a continuous log, i.e. without taking fluid samples.
Nuclear Magnetic Resonance ("NMR") well logging tools capable of determining the liquid contents of pore volume within a reservoir, and a method to use these tools are described in, for example, U.S. Pat. Nos. 5,309,098, 5,291,137, 5,280,243, 5,212,447, 4,717,878, 4,717,877, 4,717,876, and 4,710,713. In particular, U.S. Pat. No. 5,291,137 discloses a Carr-Purcell-Meiboom-Gill ("CPMG") pulse sequence and echo response and a method to obtain free fluid porosity, total NMR porosity, bound fluid porosity, spin-spin relaxation time (which is related to pore size distribution in sandstone, and continuous permeability logs. Recovery times between CPMG pulse trains is typically between 0.5 and 1.5 seconds. Because the relaxation time of methane in typical reservoir conditions is greater than three seconds, the total NMR porosity measured in this method could not include the volume occupied by hydrocarbon gas.
Recently, a new logging tool, the MRIL (TM NUMAR Corp. of Malvern, Pa.) has been introduced for determining the liquid-filled porosity in a lithology independent manner, i.e., the tool response does not require a lithology correction to determine porosity. The MRIL uses pulsed nuclear magnetic resonance of the mobile protons in the pore space. However, according to the tool's manufacturer, this tool is not capable of measuring the hydrocarbon gas content of the pore space. For example, Chandler et al. of NUMAR in SPE 28635 "Improved Log Quality with a Dual-Frequency Pulsed NMR Tool" state that "Gas causes an uncorrectable reduction of MRIL porosities. This "gas effect" is a characteristic of all NMR logging tools."
This "gas effect" is a major disadvantage because the main or only hydrocarbon in many reservoirs is natural gas. The pore fluids in these reservoirs consist of only brine and natural gas. The inability to measure gas means that the NMR logging tool will not measure gas-filled porosity and must rely on comparison with other logging tools, such as density and neutron logs, to measure porosity,
It is therefore an object of the present invention to provide a method to determine the pore volume occupied by hydrocarbon gas in subterranean formations using a nuclear magnetic resonance log wherein formation properties, other than pore pressure and temperature, do not have to be known. In another aspect of the present invention it is an object to determine the hydrocarbon gas content of a formation wherein the gas content can be determined without knowledge of the properties of the formation rock.